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FREDERICKSBURG, Va. — Army divers set off 600 pounds of plastic explosives Monday and breached a 94-year-old dam on the Rappahannock River to enable fish to swim upstream once more.

The first blast beneath the Embrey Dam made a loud cracking noise followed by a burst of smoke. A second explosion shook the city, spewed debris and sent a wave rolling downriver.

The $10 million project calls for the 22-foot-high dam to be removed by February 2006.

Residents began arriving before dawn to watch. For many, the demolition was sentimental, recalling an industrial era when the riverbanks were dotted with textile and grain mills.

“It’s sort of out of respect for the dam,” Bob Wallace said. “It’s done its job well. It’s a landmark.”

The demolition will make the Rappahannock the longest free-flowing river in the Chesapeake Bay watershed and should also open up hundreds of miles of river to migratory fish — including American shad, hickory shad and blueback herring — for the first time since 1854, when a wooden crib dam was built to power mills.

The Embrey Dam has not produced power since the 1960s.

Fishermen will see results almost immediately when spawning season begins next month. Boaters will have to wait up to two years while chunks of concrete and steel are removed from the channel.

In 1999, the Edwards Dam on Maine’s Kennebec River was torn down to let fish swim upstream again, becoming the first hydroelectric dam in the country removed by the U.S. government against its owners’ wishes.